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Black Uniformity of Monitors

What it is: Evenness of blacks.
When it matters: Dark scenes viewed in a dark room.
Score components:

Black uniformity refers to a monitor's ability to display solid black over the entirety of its screen. A monitor with perfect black uniformity does not produce distracting clouding or glow in the corners of the screen and is free of backlight bleed (sometimes also called flashlighting). This helps letterboxes on cinematic content look uniform and makes shadows and dark parts of the screen even and free of blotches. It is most noticeable when the monitor is used in a dark room and unfortunately, every standard LED/LCD monitor will have some form of uniformity issues.

For this test, we take a picture of a monitor showing our black uniformity test pattern, which mostly consists of pure black. We then process this picture and calculate the standard deviation between the different areas of the screen which are supposed to display black.

Test results

When it matters

Monitor with significant uniformity issues Poor black uniformity (3.875%)
Good black uniformityGood black uniformity (0.623%)

Most black uniformity issues like clouding or backlight bleed (BLB) are not usually visible in a bright room. This means that the black uniformity of your monitor isn't very important unless you're using your monitor in a dark environment.

It is actually quite rare for monitors to have good black uniformity, and any LCD will at least have some. In general, the effect is often worsened by their somewhat low average contrast ratios.

Our tests

Native Black Uniformity Picture

Our black uniformity test begins with a picture which aims at capturing the imperfections that a monitor produces while trying to display a uniform black across its screen. It is the easiest and most useful way to assess the evenness of the blacks produced by a monitor.

To take our photo, we first display our test pattern on the monitor. This pattern includes a white cross in its center which helps counteract a few tricks that some manufacturers can use to make monitors look like they perform better than they actually can.

The screen is then set to display the center point of this cross at our standard 100 cd/m2, and a picture is taken at F4.0, ISO-200 with a 2s shutter speed.

These camera settings were chosen as they were found to be the most representative of what you see if using the monitor in a pitch black room.

Black uniformity test patternOur black uniformity test pattern

Native Black Uniformity Std. Dev.

What it is: Average of the squared difference of the blacks.
When it matters: Dark scenes viewed in a dark room.
Good value: <2%
Noticeable difference: 1%, but keep in mind that it varies a lot by unit, even of the same model; yours likely will not end up exactly like ours.

To score the uniformity of each monitor as objectively as possible, we calculate the standard deviation between every pixel in the picture. The more variance there is, the worse the black uniformity. This calculation produces a percentage value which represents the average fluctuation in color produced by the monitor while displaying black. The lower the number, the more uniform the blacks produced by this monitor are.

Black Uniformity with Local Dimming Picture

Our Black uniformity with local dimming picture is done with the same parameters as our normal black uniformity picture, but we also enable the monitor's local dimming feature and set it to maximum. The cross pattern helps to display how well a monitor's local dimming system can dim the surrounding areas without dimming the cross or leaving a visible halo around the pattern.

Black Uniformity with Local Dimming Std. Dev.

What it is: Average of the squared difference of the blacks with Local Dimming enabled
When it matters: Dark scenes viewed in a dark room.
Good value: <2%
Noticeable difference: 1%, but keep in mind that it varies a lot by unit, even of the same model; yours likely will not end up exactly like ours.

This test result is calculated using the same process as our normal black uniformity picture, but it is done instead on our Black uniformity with local dimming picture. Oftentimes, the limited precision of the local dimming system the monitor is equipped with will make the uniformity worse than with the feature disabled.

Additional information

What causes backlight bleed and clouding?

Backlight bleed and clouding can occur for a few reasons:

  • Poorly designed backlight. LCD monitors require a backlight (nowadays made of LEDs) which produces the light required to show a decent picture. The position and quality of these backlight systems have a big impact on the uniformity of the screen. Most monitors today only have a strip of LEDs at the top or bottom of the screen and use a series of diffusion films behind the LCD panel to create an even backlight. Unfortunately, not all of these designs are made the same, and edge-type backlights will often have visible backlight that looks like a distracting bright strip on one of the edges of the screen.
  • Lenient manufacturing and uneven frames. LCD screens are made of many layers, and most of these are quite flexible. When the monitor is being built, it is likely that some of these layers sit in the frame of the monitor with a few minor bends. These can cause uneven diffusion of light which leads to clouding and backlight bleed.
  • Uneven intensity of the lighting. Sometimes, the LED themselves have manufacturing issues. This leads to certain regions of the screen having stronger backlight than others which leads to very visible uniformity issues in blacks but also colors and greys.

What is IPS Glow?

IPS glow refers to a specific type of uniformity issues that, as the name suggest, is most common with IPS-type LCD monitors. Unlike normal uniformity issues, the cause for IPS glow is mostly the screen's vertical viewing angle, which is why it often appears at the corners.

In a normal viewing position, with your eyes being level with the center of the screen, the corners of your monitor are at a much steeper angle than the central areas. These areas of the screen can start to show the artifacts that appear when using your monitor beyond its viewing angle. Colors and brightness shift and parts of the screen with very slight backlight bleed have the effect exacerbated by a bright amber hue.

Unfortunately, not much can be done to mitigate IPS glow except adjusting your viewing position. The easiest way to assess that the uniformity issues on your monitor are due to IPS glow and not normal clouding is by moving on a vertical axis while looking at your monitor. If the amber clouds shift around as you move, you are probably seeing IPS glow.

Monitor with Visible IPS Glow
Monitor with visible glow
Black uniformity test patternIPS Glow is caused by the viewing angle of the monitor. Your angle of vision for the corners of the screen is much steeper than at the center, and it gets worse the closer you sit to your screen.

How to get the best results

The easiest way to reduce the appearance of uniformity issues is by sitting directly in front of your monitor, as sitting off-axis tends to worsen the effects significantly. Otherwise, the most effective way to fix more distracting uniformity problems is to simply return or replace your monitor. There are some ways to enhance it slightly, but more problematic cases can rarely be solved with these techniques. Uniformity will change across every single monitor, so it is worth trying out multiple units of the same model. If the problem persists, it is likely that this model, in particular, can be problematic.

Backlight bleed can sometimes be reduced if it is caused by the outside frame of your monitor sitting unevenly. If your monitor has rear-accessible screws to disassemble the frame, you can try tightening or loosening them. This can have a very strong impact, so be careful with your adjustments as it also can make the issue worse.

If you have minor clouding, you can also try a common technique which occasionally helps. With the monitor on and displaying either a black frame or our test pattern shown above, look for the brighter areas of the screen. Using a soft cloth (one you would use to clean your screen), gently massage the brighter spots. It might take a while, but it can be quite effective. Make sure to be quite gentle though, as pressing too hard can obviously damage your monitor.

Other notes

  • Every LCD monitor will have some form of uniformity issues.
  • Physical impacts can affect uniformity, but it otherwise doesn't get worse over time.
  • The larger the screen, the more chances you have of running into uniformity problems.
  • If you often use your screen in the dark, ambient lighting behind your screen can greatly reduce the visibility of uniformity issues and greatly enhance perceived contrast.
  • As your monitor heats up and gets used for the first few weeks, it can get better as internal parts settle in properly. Sometimes, just a little time can help parts that were moved in shipping get placed properly.

Conclusion

Black uniformity describes a monitor's ability to evenly display blacks over different regions of its screen. It is an important factor of picture quality in a dark room, but some cases can cause dark content to look blotchy and uneven in brighter viewing environments. We take a picture of a monitor showing a (mostly) black frame in a pitch black room to easily visualize the uniformity of each monitor, then process this picture and calculate the standard deviation across the screen to objectively determine its evenness.

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